The air column in the trachea can be seen throughout its length as it descends obliquely inferiorly and posteriorly. The course of the trachea on a normal lateral view is straight, or bowed anteriorly in patients with aortic unfolding, with no visible indentation from adjacent vessels. Small indentations into the air column of the trachea from tracheal cartilage rings may be apparent on the lateral view. The carina cannot be identified on the lateral view (though the right main bronchus is often mistaken for it). Its anterior wall is visible in a minority of patients, but the posterior wall is usually seen because lung often passes behind the trachea, thereby permitting visualisation of the posterior tracheal (stripe) band. The thickness of this stripe is 2–3 mm, provided it is formed solely by the tracheal wall and pleura (Fig.1). If a large amount of air is present in the oesophagus, the posterior tracheal band may be much thicker, since it then comprises the combined thicknesses of the posterior tracheal wall and the anterior oesophageal wall. Alternatively, the lung may be separated from the trachea by the full width of a collapsed oesophagus, leading to a band of density measuring 10 mm or more (Fig. 1).

FIGURE 1 ■ Lateral view of trachea and major bronchi. (A) In this example, the posterior wall of the trachea is outlined by lung posterior to it (arrow). (B) In this example, the collapsed oesophagus is between the lung and the trachea (arrow).

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